The present invention relates generally to an electrophysiology diagnostic device employed during the implantation of an implantable cardioverter-defibrillator (ICD). More specifically, the present invention relates to a rapid response voltage threshold determination circuit used in an electrophysiology diagnostic device.
A healthy heart beats approximately 100,000 times a day and pumps about five quarts of blood each minute. The heart beat is regulated by electrical impulses or signals, which signals originate in the heart's natural pacemaker, the sinoatrial (SA) node. The SA node, which is located in the right atrium, produces very small electrical impulses that vary in rate depending upon the body's demands for oxygen and nutrients. Typically, the SA node controls the heart rate in the range of 60 to 80 heart beats per minute (bpm) in an average person at rest. These signals first cause the atria to contract and are then routed to the ventricles, causing the latter to contract, via the atrioventricular (AV) Node.
If the heart's own electrical signal is interrupted, delayed, or stopped, heart rhythm disturbances, i.e., arrhythmia, can result. Cardiac arrhythmia is classified as either bradyarrhythmia or tachyarrhythmia. In bradyarrhythmia or bradycardia, the heart rate is too slow to meet the body's demands; in tachyarrhythmia, the heart rate is fast but inefficient so that the heart cannot meet the body's blood circulation demands. Tachyarrhythmia is further subdivided into tachycardia and fibrillation. Tachycardia generally signifies a resting heart rate in excess of 100 bpm while fibrillation signifies a heart beat which is so fast and random that blood circulation, for all practical purposes, stops. Tachyarrhythmia, particularly ventricual fibrillation, is a life threatening condition.
A recently introduced medical device, the implantable cardioverter-defibrillator (ICD), delivers bradycardia pacing and tiered therapy, i.e., three different therapeutic electrical impulses to the heart: antitachycardia pacing; cardioversion; and defibrillation. ICDs are electrical devices, about the size of a deck of cards, that are attached to one or more leads or wires inside or outside of the heart, and are implanted into either the patient's chest or abdomen. These devices can detect and treat very fast, lethal heart rhythms by either shocking the heart or pacing the heart back to a normal rhythm. Most ICD's utilize diagnostic algorithms base primarily on sensed heart rate to identify tachyarrhythmias. For example, a device may be programmed to identify tachyarrhythmia for a sensed heart rate of 180 bpm or greater. If it senses that the average heart rate for a predetermined number of intervals is greater than or equal to 180 bpm, it will initiate either anti-tachycardiac pacing or a high voltage (HV) cardioversion pulse according to its programmed parameters. Many ICDs also offer electrogram (EGM) storage of arrhythmia events treated by the ICD.
ICD implantation requires a surgical procedure which has evolved over time. The most common technique currently used includes a non-thoracotomy, lead alone approach which involves insertion of one or more transvenous leads into the subclavian vein in the shoulder after which the lead is advanced into the right ventricle of the heart. The proximal end of the lead is attached to the ICD placed in the chest (like a pacemaker). If needed, a small subcutaneous patch electrode may be implanted under the skin on the left lateral chest wall.
The output stage 100 of a typical ICD is illustrated schematically in FIG. 1. The illustrated resistance-capacitance (RC) circuit consists of a capacitor C.sub.0 for supplying a controlled electrical HV pulse to the load resistance R.sub.L, i.e., the patient's heart and the leads connected thereto, via a switch assembly S.sub.0, which is shown as a double pole, double throw (DPDT) type switch assembly. It will be appreciated that the electrical pulse delivered to the load R.sub.L exhibits a high voltage peak soon after switch assembly S.sub.0 is closed, followed by exponential decay of the voltage through load resistance R.sub.L. Many advanced ICDs apply a so-called biphasic pulse to load resistance R.sub.L, as shown in FIG. 2, by reversing the leads running to the load resistance R.sub.L during the pulse period. It will be appreciated that V.sub.1i and V.sub.1f and that V.sub.2i and V.sub.2f in FIG. 2 refer to the initial and final voltages of pulses P.sub.1 and P.sub.2, respectively, of pulse widths PW.sub.1, and PW.sub.2. The specified pulse shape can also be derived by switching between two capacitors located in the ICD, as discussed below. Hereinafter, the delivered, exponentially decaying HV electrical pulse shall be referred to using the generic term "HV pulse" irrespective of the output waveform.
It will also be appreciated that the energy E stored by the capacitor C.sub.0 is given by the expression: EQU E=0.5*C.sub.0 *V.sup.2 (1)
where E is the energy in joules, C.sub.0 is the capacitance in farads (F) and V is the voltage in volts. Pacing pulses (delivered from a separate, independent output stage) are normally in the range of micro--to millijoules; cardioversion pulses generally range between 1.0 and 5.0 joules. Defibrillation therapy applies HV pulses delivering from 5.0 to 50 joules to the patient's heart per pulse.
During implantation of the ICD device, two parameters are typically determined. First, the load resistance R.sub.L, i.e., heart impedance and lead resistance, is determined. Load resistance R.sub.L must be assumed to be an unknown at first since it is a complicated function of a given patient's heart and the ICD lead placement. Moreover, since it should be assumed that the patient impedance may vary somewhat from shock to shock due to possible repositioning of the leads, i.e., changes in the electrode/tissue interface, patient safety considerations suggest that a determination of R.sub.L should be made via a measurement at the beginning of or immediately following each HV pulse event. For example, the electrophysiology apparatus described in commonly assigned U.S. Pat. Nos. 5,115,807, 5,014,697 and 4,827,936, which patents are incorporated herein by reference for all purposes, disclose that the load resistance R.sub.L is not determined until the HV pulse event has been completed. Since the amount of energy delivered to a patient and the rate of change in the HV pulse energy are greatest during the first portion of the HV pulse exponential decay, the patient impedance should be determined as soon as possible in order to limit energy delivery to the patient's heart in excess of that needed for defibrillation. Conventional measurement systems in such electrophysiology apparatuses permit as much as a 25 Volt error between the desired HV pulse and the actual HV pulse before corrective measures to limit the energy applied to the heart are initiated.
Although the size of the capacitor C.sub.0 in the ICD output stage 100 is known, new ICD models are being released every year and they may have different capacitor values. It will be appreciated that the size of the capacitor C.sub.0 will impact the useful life of the power supply incorporated into the ICD, i.e., the smaller the capacitor, the longer the power supply will last. However, the smaller the capacitor, the less energy it can deliver and, consequently, the less safety margin it can provide. For that reason, a number of ICD devices, each with a different capacitance, may be provided to the physician for selection during the implantation procedure with the optimum device being selected based on measured defibrillation thresholds (DFTs). Since the electrophysiology diagnostic device should provide at least a corresponding number of exponentially decaying HV pulse waveforms for accurate DFT testing, it is generally accepted that the electrophysiology diagnostic device will contain either a variable capacitor or a selection of fixed capacitors corresponding in size to the capacitors in the available ICD device models, which fixed capacitors can be installed in the electrophysiology diagnostic device one at a time. However, since variable capacitors are generally not available in the 50-150 microfarad (.mu.F) range, particularly with the compact footprint and low equivalent series resistance (ESR) needed for the electrophysiology diagnostic device, the conventional solution is to use replaceable, interchangeable capacitors. It will also be appreciated that since the capacitors used in ICD devices are fast discharge capacitors, e.g., photo-flash capacitors, the cost for providing several interchangeable capacitors for use in the electrophysiology diagnostic device may be prohibitively expensive due to the custom nature of the capacitors, based on the limited number of sizes of suitable, commercially available capacitors.